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What is Gravity?

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About 100 years ago, Albert Einstein (1879-1955) began to explore anew the ... Einstein eventually realized that these objects all followed the same path ... – PowerPoint PPT presentation

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Title: What is Gravity?


1
What is Gravity?
"Colliding Black Holes"CreditNational Center
for Supercomputing Applications (NCSA)
  • Fred Raab,
  • LIGO Hanford Observatory

2
Gravity in history
  • Gravity is the phenomenon with which mankind has
    the most experience.
  • Aristotle believed that gravity pulled down
    heavier objects more than lighter ones because
    heavier objects were more earthlike
  • Aristotelian thought dominated history until the
    Renaissance

3
Renaissance in gravity
  • Galileo (1564-1642) made key discoveries in study
    of gravity and motion
  • Discovery of relativity of motion concept that
    motion can only be determined relative to a frame
    of reference
  • Discovery that the motion of a falling object was
    described by a simple mathematical rule that did
    not depend on what was falling
  • Isaac Newton (1643-1727), the master of motion,
    codified Galilean relativity into his laws of
    mechanics
  • space and motion were defined relative to a
    reference frame
  • time was an absolute quantity ? all clocks ticked
    at the same rate
  • law of universal gravitation ? action at a
    distance

4
Fast Forward 200 Years
  • About 100 years ago, Albert Einstein (1879-1955)
    began to explore anew the concepts of space and
    time
  • Only two forces were known at that time
    electromagnetism gravity
  • Theory of electromagnetism had correctly
    described motors, generators, radio and light
    transmission
  • Newtons mechanics had correctly described motion
    of neutral matter, including an accurate
    description of how gravity acts, but not how it
    arises
  • But the two theories were incompatible in the
    description of rapidly moving electrical charges
  • Einstein eventually discovered that the
    incompatibility was caused by the presence of
    absolute time
  • Two travelers in relative motion, not only cannot
    define who is moving, if they move fast enough
    they also disagree on the time between events

5
Special Relativity
  • Einsteins Special Theory of Relativity (1904)
    described motion of matter in the absence of
    gravity
  • Everything is relative, except the speed of
    light, which is absolute, in agreement with
    electromagnetism
  • Speed of light is a property of space
  • There is a special symmetry between space and
    time, mass and energy
  • All observers in relative motion observe the same
    laws of physics but they disagree on the
    definitions of time and space used to describe
    those laws
  • Theory was controversial at the time
  • Nobel committee did not mention Relativity in
    Einsteins Nobel Prize citation, which was for
    the explanation of the photoelectric effect
  • Predictions
  • Moving rulers contract
  • Moving clocks tick slower
  • Matter is just a frozen form of energy (e.g.,
    nuclear power industry)

6
The Dawn of General Relativity
  • Einstein struggled another decade to discover how
    to incorporate gravity into relativity
  • A big clue was known by Newton and has often been
    observed by us since the dawn of the space age
  • Microgravity as we view it on video of
    astronauts aboard the Space Shuttle or the Space
    Station was predicted by Newtons law of
    universal gravitation, which showed that all
    objects fall exactly the same under gravitation,
    and confirmed by Newtons pendulum experiments
  • Einstein eventually realized that these objects
    all followed the same path through space and time
    and that gravitation was the consequence of
    geometrical properties of space and time

7
The Essence of General Relativity
  • Space and time are things, not concepts
  • Space and time affect the motion of objects and
    objects affect the character of space and time
  • Things move along the shortest path through the
    four dimensions of space and time
  • The presence of matter (or energy) warps space
    and time, changing their geometrical properties
  • The paths of all objects naturally follow the
    curvature of space and time
  • In flat spacetime, General Relativity reduces to
    Special Relativity

8
Mass Warps Space, Affecting Paths of Objects and
Light
  • Presence of mass gives space the appearance of
    lumpy glass as evidenced by the bending of light
  • First observed during the solar eclipse of 1919
    by Sir Arthur Eddington, when the Sun was
    silhouetted against the Hyades star cluster

A massive object shifts apparent position of a
star
  • Einstein Cross
  • Photo credit NASA and ESA

9
Was Einstein Right?
  • Remarkably so, but it took time to appreciate how
    right he was!
  • Checking relativity needed good rulers and
    clocks, and measurements over large expanses of
    space and time
  • Golden age of experimental relativity began in
    late 1950s
  • Radar had become an excellent ruler
  • Atomic timekeeping came of age
  • Space flight allowed controlled measurements over
    large distances
  • Dozens of new effects predicted by relativity
    have been confirmed to accuracies between 0.1 to
    0.0001
  • No test of general relativity has ever found a
    discrepancy!

10
The Frontier of Relativity Gravitational Waves
  • Gravitational waves are ripples in space when it
    is stirred up by rapid motions of large
    concentrations of matter or energy
  • Rendering of space stirred by two orbiting black
    holes

11
Emission of Energy by Gravitational Waves Has
Been Observed
Emission of gravitational waves
  • Neutron Binary System Hulse Taylor
  • PSR 1913 16 -- Timing of pulsars

17 / sec


8 hr
  • Neutron Binary System
  • separated by 106 miles
  • m1 1.4m? m2 1.36m? e 0.617
  • Prediction from general relativity
  • spiral in by 3 mm/orbit
  • rate of change orbital period

Hulse, Taylor receive Nobel Prize
12
Basic Signature of Gravitational Waves for All
Detectors
13
The Laser Interferometer Gravitational-Wave
Observatory
LIGO (Washington)
LIGO (Louisiana)
Brought to you by the National Science
Foundation operated by Caltech and MIT the
research focus for more than 500 LIGO Scientific
Collaboration members worldwide.
14
Is General Relativity the final word on gravity?
No!
15
What was going on in the subatomic world
  • Spurred on by Einsteins early work, quantum
    mechanics happened
  • Einstein confirmed reality of atoms
  • Einstein confirmed that energy could only be
    exchanged in units of quanta
  • Quantum mechanics was birthed (principally due to
    the work of others) from these basic ideas
  • Quantum mechanics explains the structure of
    atoms, the emission and absorption of light and
    the stability of matter
  • New forces (nuclear force, weak force) are
    discovered
  • Eventually all forces (electromagnetism, nuclear
    and weak) are shown to obey quantum laws and it
    becomes understood that they were all of
    comparable strength in the conditions that exited
    in the early universe

16
What is not talked about in polite company
  • General Relativity gives a beautiful and accurate
    description of the universe on scales extending
    from macroscopic objects to the edge of the
    visible universe and back to the age when atoms
    first formed
  • GR has passed many experimental and observational
    tests and never failed
  • Quantum Mechanics gives an extremely accurate
    description of the universe on scales comparable
    to and smaller than an atom
  • QM has passed many experimental tests of
    incredible accuracy (up to 12 decimal places!)
    without failure
  • Unfortunately these two theories are incompatible

17
Incompatibilities of GR and QM
  • QM describes an indeterminate world ruled by
    chance whereas GR is determinate
  • QM objects can only have discrete energy levels
    and exchange quanta of energy GR objects have a
    continuum of energy levels and can exchange
    energy smoothly
  • In the early universe and in the interiors of
    black holes, the subatomic and the cosmic scales
    become the same, so a single set of laws must
    apply GR and QM must be approximations that
    apply as the universe evolves

18
A popular contender string theory
  • Basic idea is that the objects we call particles
    and the exchange particles that mediate the
    forces of nature are actually made up of tiny
    vibrating strings
  • By making particles of strings (which have some
    spatial extent) rather than of points, one avoids
    the troublesome infinities that crop up
  • The same pieces of string can emulate different
    particles depending on how they vibrate
  • To have the mathematics reproduce the richness of
    particles, forces and symmetries in nature
    requires that the strings be able to vibrate in
    many dimensions

19
Extra dimensions? What extra dimensions?
  • The most popular variants of string theory
    require at least 10 spatial dimensions
  • But if there are 10 spatial dimensions how come
    we only see 3?
  • One possibility is that the extra dimensions are
    small and rolled up think about a human hair
  • Another possibility is intriguing
  • Suppose the electrical, weak and strong forces
    only act in 3 spatial dimensions, but gravity can
    leak out into the extra dimensions
  • Since almost all our information about the
    universe has come from electromagnetic signals,
    why would we know about the other dimensions?
  • Furthermore, the leakage of gravity into the
    other dimensions might explain why gravity is
    weak compared to the other 3 phenomena

20
Should we believe in string theory?
No!
  • A scientific theory must make falsifiable
    predictions, subject to experimental or
    observational testing!
  • Until string theory makes unique, falsifiable
    predictions that are confirmed, it is merely a
    mathematical model, not science.
  • A few predictions from string theory that might
    be observable
  • Sparticles might produced in the next generation
    of high energy colliders
  • Cosmic strings might be detectable by
    gravitational wave detectors like LIGO

21
Recommended Reading
  • Was Einstein Right, by Clifford Will. A bit
    ancient (more than 10 years old) accounting of
    progress in verifying Einsteins theory.
  • Black Holes and Time Warps Einsteins
    Outrageous Legacy, by Kip Thorne. From basics to
    time travel in a long, but rewarding read.
  • Black Holes, Wormholes and Time Machines, by
    Jim Al-Khalili. Some of the most challenging
    concepts of modern physics described in a
    humorous and plain-spoken style.
  • Einsteins Unfinished Symphony, by Marcia
    Bartusiak. A highly readable account of the
    search for gravitational waves.
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